An image sensor generally includes an array of pixel cells. Each pixel cell includes a photo-conversion device for converting light incident on the array into electrical signals. An image sensor also typically includes peripheral circuitry for controlling devices of the array and for converting the electrical signals into a digital image.
FIG. 1 is a top plan view block diagram of a portion of a typical CMOS image sensor 10, while FIG. 2A is a top plan view of one pixel cell 20 and FIG. 2B is a schematic diagram of the pixel cell 20. Image sensor 10 includes an array 11 of pixel cells 20. The pixel cells 20 are arranged in columns and rows (not shown). The array 11 includes pixel cells 20 in an active array region 12 and pixel cells 20 in a black region 13. The black region 13 is similar to the active array region 12, except that light is prevented from reaching the photo-conversion devices of the pixel cells 20 in the black region 13 by, for example, a metal layer, a black color filter array, or any opaque material (not shown). Signals from pixel cells of the black region 13 can be used to determine the black level for the array 11, which is used to adjust the resulting image produced by the image sensor 10.
FIG. 2A is a top plan view of a conventional pixel cell 20. FIG. 2B is a schematic diagram of the pixel cell of FIG. 2A. As is known in the art, a pixel cell 20 functions by receiving photons of light and converting those photons into charge carried by electrons. For this, each one of the pixel cells 20 includes a photo-conversion device 21, which is shown as a photodiode, but can be a photogate, photoconductor, or other photosensitive device. The photodiode 21 includes a photodiode charge accumulation region 22 and a p-type surface layer 24.
Each pixel cell 20 also includes a transfer transistor 27 for transferring charge from the photodiode charge accumulation region 22 to a floating diffusion region 25 and a reset transistor 28, for resetting the diffusion region 25 to a predetermined charge level, Vaa-pix, prior to charge transfer. The pixel cell 20 also includes a source follower transistor 29 for receiving and amplifying a charge level from the diffusion region 25 and a row select transistor 26 for controlling the readout of the pixel cell 20 contents from the source follower transistor 29. As shown in FIG. 2A, the reset transistor 28, source follower transistor 29 and row select transistor 26 include source/drain regions 60.
Several contacts 23 provide electrical connections for the pixel cell 20. For example, as shown in FIG. 2, a source/drain region of the reset transistor 28 is electrically connected to an array voltage source terminal, Vaa-pix through a first contact 23; the gate of the source follower transistor is connected to the floating diffusion region 25 through a second contact 23; and an output voltage Vout is output from the pixel cell 20 through a third contact 23.
Referring again to FIG. 1, after pixel cells of array 11 generate charge in response to incident light, electrical signals indicating charge levels are read out and processed by circuitry 15 peripheral to array 11. Peripheral circuitry 15 typically includes row select circuitry 16 and column select circuitry 17 for activating particular rows and columns of the array 11; and other peripheral circuitry 18, which can include analog signal processing circuitry, analog-to-digital conversion circuitry, and digital logic processing circuitry. Peripheral circuitry 15 can be located adjacent to the array 11 as shown in FIG. 1.
In order to obtain a high quality image, it is important that the peripheral circuitry 15 not interfere with the pixel cells 20 of the array 11. During operation, the peripheral circuitry 15 can generate charge carriers, e.g., electrons. If the peripheral circuitry 15 is adjacent to the array 11, electrons generated by the peripheral circuitry 15 can travel to and interfere with array pixel cells 20, especially those pixel cells 20 on the edges of the array 11 adjacent the peripheral circuitry 15. The interfering electrons are misinterpreted as a true pixel signal and image distortion can occur.
Another problem encountered in the conventional image sensor 10 is interference from the active array region 12 with the black region 13. When very bright light is incident on active region 12 pixel cells 20 adjacent to the black region 13, blooming can occur and excess charge from the active region 12 pixel cells 20 can travel to and interfere with pixel cells 20 in the adjacent black region 13. This can cause inaccurate black levels and distortion of the resultant image.
Accordingly, it would be advantageous to have an improved image sensor with reduced interference.